Isotope separation by means of laser irradiation has been extensively discussed and is currently the subject of intense interest, especially with regard to the separation of uranium isotopes. This work has been hampered, however, by the lack of a suitable tunable laser working in the relevant frequency range (near 16 microns).
One work that has come to our attention involves a nonresonant three-photon mixing process .omega..sub.3 = .omega..sub.1 - .omega..sub.2, where .omega..sub.1 is generated by a CO laser and .omega..sub.2 is generated by a CO.sub.2 laser (U.S. Pat. No. 4,011,462). This work suffers from the drawback that the output frequency cannot be varied continuously.
Another previous work is a physical measurement of the exciton structure of CuCl that involved a different four-photon optical mixing process in which all of the input frequencies were in the very near infrared ("Interference of Third-Order Light Mixing and Second-Harmonic Exciton Generation in CuCl," Physical Review, 9, 1853 (1974)). This process involved the use of only two input frequencies (.omega..sub.3 = 2.omega..sub.1 - .omega..sub.2) and was therefore limited to the low power afforded by single resonance enhancement. This method would also have difficulty in providing phase-matching in the frequency range of interest, which is quite far from the visible.